Within the framework of the present invention, it is considered that a time constraint is a constraint which requires a given arrival time, of RTA (“Required Time of Arrival”) type, at a particular waypoint of the flight trajectory followed by the aircraft.
The management of a time constraint along a flight plan followed by an aircraft generally presents drawbacks which limit the field of application of the various solutions implemented for complying with this time constraint.
Specifically, to compensate for an advance or a delay with respect to a scheduled timetable, a speed modulation is generally implemented. It is also possible to envisage lengthening or shortening the flight trajectory followed. However, certain parts or sections of the flight trajectory, along the flight plan followed, are themselves constrained by the speed, that is to say depend on the speed. In particular, the vertical trajectories, especially a climb or descent (vertical) profile, generally depend on the speed of the aircraft.
Hence, the modulation of the speed during such a climb (or descent) phase leads to a modification of the slopes flyable by the aircraft and therefore involves a choice of strategy, namely:                according to a first strategy, try to fly along predetermined climb and/or descent profiles, with speeds other than those for which these profiles have been determined; or        according to a second strategy, recalculate the climb and/or descent profiles as a function of the new speeds, obtained following the speed modulation.        
The choice of one of these strategies does not involve appreciable consequences during an aircraft climb phase, other than the displacement (in time or in distance) of the point of arrival in the cruising phase. With regard to a descent phase, the consequences are different depending on whether the aircraft is already or is not yet descending, upon implementing the chosen strategy. When the aircraft is not yet descending, the point at which it begins descending moves ahead of the aircraft so that the consequences are slight.
On the other hand, when the aircraft is near to or on the descent profile, a speed modulation (in accordance with the first aforementioned strategy) tends to move the aircraft away from this profile, and this may run counter to the desired effect relating to the upholding of the time constraint. Furthermore, a new calculation of a profile (in accordance with the second aforementioned strategy) involves a new relative position of the aircraft above or below this new profile, and therefore corrective actions so as to converge towards this new profile, which corrective actions run counter to compliance with the time constraint.
These various problems and the drawbacks in solving them (algorithmic complexity, complexity of the guidance laws, overconsumption of fuel, passenger discomfort, etc.) have led aircraft manufacturers and equipment manufacturers to exclude, at least partially, the descent profile from the field of application of the solutions implemented for ensuring the upholding of a time constraint. This generally results either in the exclusion or the prohibition of time constraints on the descent and approach phases, or in a more or less significant reduction in the aircraft's abilities to compensate for an advance or a delay as soon as it is engaged on a descent profile.
The previous limitations entail an increased risk of the aircraft not being able to comply with the time constraint and therefore, for example, missing a landing slot on a congested airport, thus involving the following of a waiting circuit, additional holdups, additional consumption, cost overheads, etc.
Additionally, it is known that the requirements to uphold time constraints are increased for military aircraft, such as military transport airplanes. In this case, the margins to be complied with are only a few seconds, and it may turn out that the aircraft has to manage up to five different time constraints. Moreover, these may be positioned anywhere in the flight plan, including along low-altitude flight sections, which are yet more constrained than the aforementioned descent phases. A method for constructing a low-altitude flight trajectory section is known from documents FR-2 870 607 and FR-2 897 154.
It is known that the low-altitude flight sections are generally formed of a succession of climb and descent segments, intended to allow the aircraft to overfly the relief as closely as possible. These sections are constrained by the speed, just like the aforementioned climb or descent phases, but two additional constraints are added to this speed constraint, namely:                the flight speed along a low-altitude flight section is a speed which is required by the crew. It exhibits an operational dimension and it should therefore only be modified (or modulated) in an extreme situation; and        the calculation of a new low-altitude flight section exhibits a high calculation time and is potentially troublesome for the crew during the conduct of the mission. Hence, the number of such calculations must be minimized.        
The object of the present invention is to remedy the aforementioned drawbacks. It relates to a method of guiding an aircraft flying along a flight trajectory comprising at least one constrained section, with which a required speed is associated. This signifies that the characteristics (slope, etc.) of this constrained section are calculated (and generally optimized) while taking account of a speed at which the aircraft is supposed to fly along said section. Moreover, the aircraft must comply with at least one time constraint which, by definition, requires a given arrival time at a particular waypoint of said constrained section of the flight trajectory.
The aim of said guidance method is to ensure a precise arrival time, to within a few seconds, at the level of said particular waypoint of the flight trajectory, while guiding the aircraft along at least one constrained section of the flight trajectory, that is to say of at least one section with which a required speed is associated.
For this purpose, according to the invention, said method according to which in the course of the flight:    A/ speed setpoints are determined which, when they are applied to the aircraft during the flight along said flight trajectory, allow it to arrive at said waypoint at said required arrival time; and    B/ during the guidance of the aircraft along said flight trajectory, said speed setpoints are applied to it,is noteworthy in that:            an auxiliary arrival time is determined, at which the aircraft must arrive at an auxiliary waypoint which corresponds to the start of said constrained section, so as to be able to comply with said time constraint, said auxiliary arrival time being determined as a function of the distance between said waypoint and said auxiliary waypoint and as a function of the required speed for said constrained section;        in step A/, auxiliary speed setpoints are determined allowing the aircraft to arrive at said auxiliary arrival time at said auxiliary waypoint representing the start of said constrained section; and        in step B/, said auxiliary speed setpoints are applied to the aircraft, upstream of said constrained section in the direction of flight (along the flight trajectory), so that at said auxiliary arrival time said aircraft arrives at said auxiliary waypoint representing the start of said constrained section and that it thus complies with said time constraint, while being able thereafter to fly at said required speed along said constrained section.        
Thus, by virtue of the invention, the time constraint is transposed to the start of the constrained section, with which a required speed is associated, so that:                on the one hand, it is possible to comply with said time constraint, the auxiliary arrival time being determined as a function of the distance between said waypoint and said auxiliary waypoint and as a function of the required speed for the constrained section, as specified below;        on the other hand, the guidance of the aircraft, which is carried out mainly with the aid of a speed modulation, so as to comply with said time constraint, is carried out upstream of the constrained section so that no speed modulation (except for implementing certain corrections) need be carried out on said constrained section so as to comply with the time constraint. Consequently, said constrained section can be flown at the required speed (for which it has been constrained).        
Thus, by virtue of the invention, to comply with the time constraint, it is not necessary as in the aforementioned standard solutions:                to recalculate the flight trajectory at the level of said constrained section; or        to fly the aircraft along said constrained section at a different speed from the required speed,thereby making it possible to remedy the aforementioned drawbacks.        